Steel sheet piles have been used since the beginning of the 20th century in the construction of quays and harbours, locks and moles, protection of riverbanks as well as excavations on land and in water, and, in general, excavation work for bridge abutments, retaining walls, foundation structures, etc.
In addition to plain sheet pile walls, sheet piles can easily be used as infill sheeting between king piles to build up combined walls (or “combi-walls”), for the construction of deep quay walls with high resistance to bending. King piles are typically either wide flange beams or cold formed welded tubes. The infill sheeting are connected to the king piles by interlocking bars (connectors).
The design of a sheet pile wall and more generally of a steel combined wall is governed by the loads acting thereon, which include applied forces from soils, water and surface surcharges. Mechanical performance of the structural elements like sheet piles and tubes is thus a primary parameter.
Another essential aspect to be considered in a combined wall design is durability. The lifetime of sheet pile structures will clearly be strongly influenced by environmental factors. Those working in a marine environment are aware that corrosion is one of the most important factors to consider in the long-term life of a structure.
Indeed, chlorides found in marine environments stimulate the corrosion process and are the principal reason for the more aggressive attacks on steel. Wind and waves combine to provide oxygen and moisture for an electro-chemical reaction and abrasion may remove any protection rust film. It may however be noted that not all salt-water environments are dangerously aggressive to steel, and not all zones along the height of the piling structure are attacked at the same rate.
In fact, the seaside portion of the sheet piling wall is exposed to six “zones”—atmospheric, splash (the atmospheric zone just above the high tide), tidal, low water, immersion and soil. The corrosion rate in each of these zones varies considerably. Generally, experience has shown that steel sheet piling in coastal marine environments have the highest corrosion rates in the splash (just above mean high water) and low water (just below mean low water) zones, corrosion rates in the atmospheric and soil areas are considered to be negligible on such piling structures.
Effects of corrosion in marine environments can be accounted for by a sacrificial steel reserve and/or protective methods (paintings, cathodic protection). However, a protective painting or concrete layer can only be applied on the non-immersed zones of the steel structure.
The addition of certain alloy elements to carbon steel also provides improved performances in some environments. As early as 1913, experimental work by the steel industry indicated that small amounts of copper would enhance the atmospheric corrosion resistance of carbon steel.
In the 1960s, the so-called “Mariner” grade was developed, and is today a well-known alternative to carbon steel for sheet piles for marine environments. ASTM standard A690 gives the chemical composition of this high strength, low alloy (HSLA) steel, which contains higher levels of copper (0.08-0.11 wt. %), nickel (0.4-0.5 wt. %) and phosphorous (0.08-0.11 wt. %) than typical carbon structural steels. Tests indicated a substantially improved corrosion resistance to seawater corrosion in the splash zone of exposed marine structures than typical carbon structural steels.
Also concerned by steel corrosion in marine environment, Corus UK, Ltd. filed a patent application on Dec. 9, 2002, published as GB 2 392 919, relating to a CrAlMo corrosion resistant steel for the production of sheet piling for marine applications. The following steel composition (by weight percent) is disclosed: carbon 0.05-0.25; silicon up to 0.60; manganese 0.80-1.70; chromium 0.75-1.50; molybdenum 0.20-0.50; aluminium 0.40-0.80; titanium up to 0.05; phosphorous up to 0.045; sulphur up to 0.045; balance iron and incidental and/or residual impurities. The aim followed by Corus was to provide a weldable corrosion resistant steel, that is especially resistant to seawater, and having following mechanical properties:                minimum yield stress of about 355 MPa;        minimum tensile strength of about 480 MPa;        minimum Charpy absorbed impact energy of 27 J at a test temperature of 0° C.        
Unfortunately, this CrAlMo steel designed for sheet piling products was never manufactured on industrial scale due to initial difficulties faced up in the continuous casting process as well as some insufficient mechanical properties. Further, tests results known to the present applicant on the above steel did not permit to achieve the alleged mechanical performances. In particular, the above CrAlMo steel showed low toughness and ductility.
It may be noted that a variety of studies and tests have been carried out in the past to determine the effects of alloy elements on the anti-corrosion properties of low alloy steels. While in general authors of such studies would observe some tendencies in the effect of a certain alloy element, with respect to a given corrosion zone and over a given period of time, conclusions were always moderate. Besides, there are many contradictory results.
As a general rule, it has to be kept in mind that the relationship between anti-corrosion properties of steel in marine environment and alloy elements is considerably different with variation of marine environment. As it is known in the art, the same alloy element's effect on the anti-corrosion of steel in the splash and immersion zones can be clearly different. In fact, a given alloy element can improve the corrosion resistance of steel in one zone, but not in another zone, or even accelerate the corrosion rate in that other zone. Further, it has been observed that whereas an increase in chromium, for example, may initially improve corrosion resistance, after a certain period of time the situation may be reversed. Also, some synergistic effects may exist between alloying elements, such synergistic effect depending of course on the concentrations, but generally not varying linearly with the concentrations.
Another type of corrosion to which metallic structures may be subject is the so-called “galvanic corrosion”. Galvanic corrosion is defined as the accelerated corrosion of a metal due to electrical contact with a more passive metal in an electrolyte. Higher electric conductivity of seawater facilitates such type of corrosion between two different types of metals that can be found in a metal structure. Hence, when designing combined walls, care should be taken not to connect carbon steel structural elements with others made of micro-alloyed steel.
More recently, attention has been drawn to a further source of corrosion generally designated as microbiologically influenced corrosion (MIC). Indeed, it has lately been proved that such a type of localized corrosion was occurring in the low water zone on steel structures in marine environment. This phenomenon is known as Accelerated Low Water Corrosion (ALWC) and is responsible for extremely high rates of corrosion.
From the above it appears that numerous factors have to be considered in the construction of combined walls in marine environments. The selected steels for the different structural elements must meet the required mechanical performances, but at the same time it is desirable that the steel has improved corrosion resistance to seawater.
Although addition of certain alloying elements can be helpful to improve corrosion resistance, it should not compromise the mechanical performances. Alloying of carbon steel must thus be made carefully to achieve desired strength and toughness, enhance resistance to corrosion in one or more zones, while not accelerating corrosion in the others, and bearing weldability and costs issues in mind.
In practice, although the acute corrosion of steel in marine environments has been a matter of concern since the 1950s, it has to be noted that the vast majority of sheet piles and tubes for use in marine environment manufactured nowadays are made from plain carbon steel.